The role of cortical granules in blocking polyspermy during the fertilization of hamster eggs is being studied in vitro. The composition of the cortical granule matrix material is being determined using light and EM cytochemical techniques. Changes occurring at the oolemma are being monitored via membrane labels; a comparison of unfertilized and fertilized eggs is being made. We are trying to determine whether cortical granule exocytosis is an integral part of the block to polyspermy in the hamster. Attempts are being made to dislodge the cortical granules from the cortical filamentous network which appears to hold the cortical granules close to the oolemma. Comparative studies are in progress on the formation, chemical characterization and role during fertilization, of the cortical granules in fish and mammals.